Effects of incoherent and coherent scattering on the exposure buildup factors of low-energy gamma rays

The effects of including incoherent and coherent scattering in a calculation of the exposure buildup factors for plane normal gamma-ray sources have been investigated by using an electron-gamma-ray shower Monte Carlo code, EGS4, for water, iron, and lead in the 40- to 200-keV range. The ''true'' buildup factors for practical uses are defined to clarify the effects of bound-electron Compton (incoherent) and coherent scattering and are tabulated for penetration depths up to 10 mfp. The pseudo buildup factor increases by including incoherent scattering and decreases by including coherent scattering. The degree of each effect varies with the atomic number of the material. The effect of incoherent scattering is large for materials of small atomic number, and the effect of coherent scattering is large for materials of large atomic number.